The invention relates to an image intensifier tube, comprising a housing which is formed by an entrance window, an exit window and an envelope portion which consists partly of a translucent, electrically insulating material, which housing accomodates an entrance screen and an electron-optical system for imaging photoelectrons on an exit screen.
An image intensifier tube of this kind is known as an X-ray image intensifier tube from U.S. Pat. No. 3,026,437 and as a brightness intensifier tube from U.S. Pat. No. 4,286,148. In order to prevent local discharge phenomena in such tubes, a glass portion of the envelope is often coated with a semiconductor material on an inner side. In image intensifiers tubes comprising a photosensitive entrance screen, discharge phenomena are liable to have an image-disturbing effect because light then emitted activates the photosensitive layer, releasing, for example photoelectrons which are imaged on the exit screen, together with image-carrying photoelectrons, and thus participate in the imaging.
U.S. Pat. No. 3,026,437 mentions chromium oxide as an example of a coating material. Known coating layers have the drawback that the layer is not transparent because it consists of, for example green chromium oxide, or that the resistance of the layer is comparatively low so that a rather large leakage current occurs which substantially increases the power required for operating the tube. Moreover, known coating layers have a comparatively large thickness and their thickness and structure are not very uniform.